1. Field of the Invention
The present invention relates to an illumination device, and is particularly suitable for an image reading apparatus.
2. Description of the Related Art
Conventionally, the following problem arises in an image reading apparatus as in an image pickup apparatus in which a subject image formed by an imaging optical system is obtained by an image pickup unit such as a film or an image sensor (image pickup element). When light of a wavelength other than a visible range, in particular infrared light or ultraviolet light, reaches the image pickup element, the color changes from the original subject image (decrease in color rendering of the obtained image).
Accordingly, as a subject illumination device used in an image reading apparatus, a subject illumination device that removes infrared light by interposing a film for absorbing infrared light between a light source and an incident portion of a main scanning end incident-type light guide is known (Japanese Patent Application Laid-Open No. 2008-028617). Other than the film, a filter for absorbing infrared light is also known as a member for absorbing infrared light.
On the other hand, as a light source, a high-brightness LED (power LED) that emits a large amount of light per element is developed especially in recent years, and its application to an image reading apparatus as a light source with better color rendering than a conventional light emitting diode (hereafter “LED”) is under study.
However, when used together with the high-brightness LED (power LED), the film-like filter as in Japanese Patent Application Laid-Open No. 2008-028617 has a problem with the heat resistance of the film material to a large amount of heat generated by the LED.
Especially, an illumination device used in an image reading apparatus for a copying machine or a combination machine that performs high-speed reading is required to have high intensity as a substitute for a conventionally used high-pressure discharge lamp, and so the corresponding LED is required to have high light emission intensity with low power consumption. For example, luminous flux emission of 50 lumens or more per element is required. When used together with the LED having such high light emission intensity, the film-like filter as in Japanese Patent Application Laid-Open No. 2008-028617 has a problem with the heat resistance of the film material.
Besides, the power LED used in this application tends to exceed 120° C. in junction temperature at rated current, and is higher in temperature than the conventional LED though it generates less heat than the high-pressure discharge lamp. Hence, peripheral parts of the LED are also required to be heat-resistant. The film-like filter as in Japanese Patent Application Laid-Open No. 2008-028617 is problematic in this respect, too.
In the case of using an infrared absorbing filter instead of the above-mentioned film-like filter, the following problem arises. Since the spectral transmittance characteristics of the infrared absorbing filter are typically not as steep as those of a dichroic filter made of a multi-layer film, many infrared components are left unabsorbed and more light in the necessary visible range is absorbed. Particularly in the case of using a high color rendering white LED developed in recent years, the ratio of infrared components is higher than that of a conventional white LED, and so there is a risk of decreasing the color rendering of the obtained image.